Conventional satellite-based positioning systems, for example, a Global Navigation Satellite System (GNSS) such as the Global Positioning System (GPS) include a GPS receiver system. An important part of the receiving system is the antenna and non-ideal behavior of the antenna is one of the significant limitations in determining position with very high accuracy. Optimally, the antenna would receive only direct signals from the satellite with very high electrical phase stability regardless of the elevation and azimuth angles of the satellite. The antenna should have means for rejecting signals that have become corrupted by reflection, diffraction and/or refraction from physical structures in the vicinity of the path (or paths) of the signals arriving at the receiving antenna. The satellites transmit towards the earth with Right Hand Circular Polarization (RHCP). The best simple receiving antenna, used by a conventional GPS receiving system, will be responsive only to RHCP signals. The response of the antenna to Left Hand Circular Polarization (LHCP) should be many decibels down from that of the RHCP over a wide angular range. This type of antenna will be referred to as a High Purity Circularly Polarized (HPCP) antenna. A good high RHCP over LHCP response corresponds to a low axial ratio, which is the magnitude of the RHCP plus the magnitude of the LHCP all divided by the magnitude of the RHCP minus the magnitude of the LHCP for a given angular position in space when the antenna is exposed to a pure Linearly Polarized EM wave. An RHCP antenna should have a high ratio of RHCP over LHCP, which corresponds to a low axial ratio. A 20 dB RHCP to LHCP ratio corresponds to an axial ratio of 1.75 dB and a 24.8 dB RHCP to LHCP ratio corresponds to an axial ratio of 1.00 dB.
Many types of circularly polarized (CP) antennas are available for consideration. Some of the widely used CP antennas types include the CP microstrip patch, helical, spiral slot radiator, crossed electric dipoles (or turnstile), crossed slots, conical spirals antennas among others. The various antennas discussed above all have various shortcomings for achieving the desired high performance GPS antenna with two outputs, RHCP and LHCP. Microstrip patch antennas are likely to be too narrow band. Helical and spiral antennas can be built for RHCP or LHCP but not for both outputs simultaneously. The turnstile antenna can be built to deal with both of the above problems but it has a very poor axial ratio in the plane of the dipoles. In fact, it is difficult to obtain a good axial ratio over a wide angular range (over the upper hemisphere) with virtually any circularly polarized antenna.
The ideal GPS antenna will receive only RHCP signals from the upper hemisphere above the horizon. To address this capability, let us now consider the difficult problem of producing a HPCP signal over a large solid angle, that is the upper hemisphere. Assume that we have a turnstile antenna and the dipoles lie in a horizontal plane. In this case the turnstile antenna can produce HPCP at the zenith and at other high angles. But closer to the horizon the radiation will be more and more cross polarized, that is the RHCP over LHCP ratio will be lower and the axial ratio will be higher. In order that the overall antenna will produce HPCP in the horizontal plane it is necessary for that dipole to produce propagating field components at a given distance (R) on the Y-axis and of equal magnitude on the X-axis at a distance of R. The radiation gain patterns of Eφ and Eθ should approximate to a high degree of accuracy the trigonometric expressions, sin(2φ) and cos(2φ) respectively. The dipole as shown in FIG. 1A has a radiation null in the direction of X as shown in FIG. 1B. This means that the turnstile antenna will produce only a linearly (horizontal) polarized signal at the horizon. Various attempts to address this disadvantage have been undertaken. One proposed solution is to droop and sometimes bend the dipoles, which provides better circular polarization at the horizon but the best axial ratios at the horizon tend to be about 6 dB. Another method of promoting creation of fields off of the end of the active dipole is by the introduction of a cup or a closed end circular waveguide. For one such configuration, the axial ratio is reported to be better than 1 dB up to 28 degree off the axis of the antenna (i.e., the Z-axis). The electrical behavior of the cup (or closed end circular wave guide) is very strongly dependant on the operating frequency and the diameter of the cup. Also disclosed in the art is a full sized dipole and a cup diameter of about 1.2 free space wavelengths at the low end of the operating frequency band. This means that many circular waveguide modes may propagate into the cup to the short circuit and back out again. In general, it is not desirable to allow multiple modes to propagate in a well-structured EM device. These antennas have a moderately large bandwidth. Other prior art references disclose a dipole, shortened to an extreme degree, operating inside of the cup and none of the modes are propagating. For one such system, the antenna operating frequencies are 0.81 and 0.89 of the waveguide cutoff frequency. All modes are evanescent. This means that the signal power is very weakly coupled to the outside free space. This leads to a very narrow band operation of the antenna and it has led to a very unusual feeding structure for the dipoles. The unbalanced feed for each dipole can lead to poor purity of CP radiation. It also appears that this antenna is difficult to manufacture and to tune to the correct frequencies.
High precision GPS surveying and geographical locating systems have reached very high levels of precision and are approaching accuracies in the sub centimeter range under ideal conditions. In many locations however there are large numbers of scatterers that prevent high accuracy from being achieved. It is therefore desirable to have available methods of reducing the effects of these scatterers. The anticipated launching of a new system of satellites will give the ground station more signals that may be used to increase the number of valid data streams which will allow even better accuracy of determination of geographical location.
In high accuracy applications the mathematical processes utilized in the GPS receiver and subsequent digital processors, determine the number of wavelengths and the number of electrical degrees between that satellite and the GPS receiving antenna phase center. It is therefore important that the GPS antenna has a phase center that stays in the same location within very small tolerances as the reception angle of a given incoming wave changes from near the horizon to the zenith. The phase center should also be independent of azimuth reception angle and be fairly independent of the frequency in use.